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Eggplant-derived biochar- halloysite nanocomposite as supports of Pd nanoparticles for the catalytic hydrogenation of nitroarenes in presence of cyclodextrin Samahe Sadjadi, maryam akbari, Bastien Leger, Eric Monflier, and Majid M. Heravi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05992 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Sustainable Chemistry & Engineering

Eggplant-derived biochar- halloysite nanocomposite as supports of Pd nanoparticles for the catalytic hydrogenation

of

nitroarenes

in

presence

of

cyclodextrin Samahe Sadjadi,*a Maryam Akbari,b Bastien Léger,c Eric Monflier,c Majid M. Heravib

a

Gas Conversion Department, Faculty of Petrochemicals, Iran polymer and

Petrochemicals Institute, 15km Tehran-Karaj Highway, Pajuhesh Science and Technology Park, Pajuhesh Boulevard, postal cod; 14977-13115, PO Box 14975-112, Tehran,

Iran.

Tel:

+98

2148666

;

Fax:

+98

2144787021-3,

Email:

[email protected] and [email protected]

b

Department of Chemistry, School of Science, Alzhara University, PO Box

1993891176, Vanak Village street, Tehran, Iran. Tel: +98 21 88044051 ; Fax: +98 2188041344

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

c

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Univ. Artois, CNRS, Centrale Lille, ENSCL, Univ. Lille, UMR 8181, Unité de Catalyse et

de Chimie du Solide (UCCS), F-62300 Lens, France.

KEYWORDS : Halloysite, Biochar, Pd nanoparticles, Hydrogenation, Cyclodextrin

ABSTRACT

A novel halloysite-hydrochar nanocomposite has been prepared and applied for the immobilization of Pd NPs to furnish an efficient catalyst for the hydrogenation of nitroarenes. It was confirmed that use of the catalytic amount of -cyclodextrin (-CD) could improve the yield of the reaction significantly. With the aim of investigation of the effect of combination of Hal and Char, Char surface modification and the way of use of CD on the catalytic activity, several control catalysts were prepared and their catalytic


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activities were compared with that of the catalyst. It was confirmed that the use of HalChar as a support was more effective than the use of each component individually. Moreover, the use of -CD in its free form was more efficient than incorporating it to the framework of the catalyst or as a capping agent. It was also found that Char in its unmodified form was more efficient than modified ones. To justify the results, a precise study was carried out by comparing the average Pd particle size and loading of each samples. It was confirmed that the Pd particle size and dispersion effectively affected the catalytic activity. Additionally, -CD amount was a key factor for achieving high catalytic activity.


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INTRODUCTION

Ever-increasing environmental concerns have resulted in much attention to the development of environmentally benign heterogeneous catalysts. In this line, (bio)Char as a carbonaceous product of the carbonization of biomass offers significant environmental merits. Chars are used extensively for fuels production, soil remediation, waste

management,

greenhouse

gas

reduction,

absorbent,

catalysis

and

supercapacitors. 1-5 Hydrothermal carbonization is one of the most common methods for the preparation of Chars and the Chars produced from a hydrothermal process are often called hydrochar. The Chars can be applied in their plain form or activated form to improve their properties and increasing their efficiencies. Moreover, they can be used with other materials to furnish composites with diverse ranges of applications.1-5

One of the natural inorganic materials with tubular morphology is halloysite, Hal, (Al2Si2O5(OH)4·nH2O) that is a dioctahedral 1:1 layered aluminosilicate with properties similar to those of kaolin. The Hal tubes with inner diameters of 10–30 nm and outer


4

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diameters of 50–70 nm have different electric charge and chemical composition on each surface.6 These physical and chemical features along with biocompatibility and high thermal and mechanical stability resulted in the extensive researches for disclosing the utility of Hal in diverse range of applications 7, ranging from catalysis to drug delivery.2, 715

Hal can be successfully used with different materials such as carbon and polymeric

compounds.16-17

Use of poly/oligo saccharides for the catalysis is a well-established topic. Among oligosaccharides, cyclodextrins (CDs) that are cyclic oligosaccharides have received much attention. The unique chemical and physical properties of CDs such as their coneshape structure with hydrophobic interior space and hydrophilic exterior, bio-compatibility and non-toxicity as well as their availability and relatively low cost render them promising candidates for various applications, ranging from catalysis to drug delivery systems. 18-20 CD can exert its catalytic activity through formation of inclusion complex with the substrate and serving as a molecular transfer shuttle.18-20 Moreover, CD can be considered as a potent capping agent for the synthesis of nanoparticles with catalytic utilities.


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Considering the importance of anilines in industry, development of efficient protocols for the synthesis of anilines through hydrogenation of nitrobenzenes has been focused. In this context, diverse range of heterogeneous catalysts have been developed.21-25 Despite considerable achievements, some challenging issues such as high loading of catalyst, long reaction time and high temperatures remained unsolved.

In continuation of our research on the development of heterogeneous catalysts,26 recently, we have focused our research on the Hal-based catalysts.27 In this line, we have recently reported the high performance of the Hal-carbon nanocomposites as catalyst supports.28 Encouraged by those promising results, herein we design a novel nanocomposite, Hal-Char, composed of Hal and hydrochar and utilized it as a support for the immobilization of Pd nanoparticles. The resulting catalytic system, Pd@Hal-Char, was applied for promoting the hydrogenation of nitroarenes in the presence of -CD. A precise study was carried out to investigate the contribution of composite components to the catalysis. Moreover, the role of -CD in the catalysis and the effect of using it in its free form and incorporated in the backbone of the catalyst on the catalytic activity were


6

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scrutinized. Additionally, using other control catalysts, the effect of surface modification of Char was investigated. To justify the results and find a relationship between the catalytic performance and the structural features, the average Pd particle size and Pd loading of all samples were obtained and compared. Finally, the recyclability of the catalyst and Pd leaching were examined.

EXPERIMENTAL SECTION Materials. Green head of eggplant was selected as an abundant domestic biomass source for the synthesis of hydrochar. In this line, the heads of eggplants were washed, dried in oven at 120 °C and powdered prior to use. All chemicals and reagents used for the synthesis of the catalyst and performing the hydrogenation reaction, including

nitrobenzene,

1-nitronaphtalene,

4-nitroacetophenone,

Hal,

N‐methylimidazole, (3‐chloropropyl) trimethoxysilane, -CD, Pd(OAc)2, sodium borohydride, toluene, ethanol and sulfuric acid were of analytical grade, purchased from Sigma-Aldrich and used without further purification.


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Characterization. The catalyst as well as the control samples were characterized using ICP-AES, TGA, XRD, BET, FTIR, ZP and TEM. ICP analyses were accomplished by using ICP analyzer (Varian, Vista-pro). The sample preparation for ICP analysis was accomplished according to the previous reports.29 Typically, a known amount of the catalyst was digested in the concentrated acidic solution. Thermogravimetric analyses (TGA) were carried out using METTLER TOLEDO thermogravimetric analysis apparatus with heating rate of 10°C.min-1 under N2 atmosphere over the range of 50 to 800 °C. XRD patterns of the catalyst and the control supports were recorded using a Siemens, D5000. Cu Kα radiation from a sealed tube. BET analyses were performed using a Belsorp Mini II instrument. The degassing process of the samples was carried out at 100 °C for 3h. FTIR spectra were

recorded

by

employing

PERKIN-ELMER-Spectrum

65

instrument.

Transmission Electron Microscopy (TEM) was performed on a Tecnai Microscope (200 kV). In detail, the heterogeneous catalysts, after into powder, were deposited onto a carbon coated copper grid. Metal nanoparticle size distributions have been determined from the measurement of around 200 nanoparticles found in arbitrarily chosen area of the


8

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images using the program SCION Image. Zeta potential measurements were carried out in water suspension using a Malvern Zetasizer Nano ZS at a controlled temperature (25±0.1°C). The -potential corresponds to the potential difference between the dispersion medium and the electrical double layer of the fluid attached to the dispersed particles. The measurements are based on a Laser Doppler electrophoretic mobility of the heterogeneous catalyst via the Helmholtz-Smoluchowski equation  = (η/ε)μe, where μe is defined as the ratio between the velocity of the heterogeneous catalyst and the magnitude under the applied electric field, η is the viscosity of the suspending liquid and ε stands for the dielectric conductivity of water. The heterogeneous catalysts were analyzed without any previous treatment. All the samples were analyzed in triplicate using the DTS Software from Malvern Instruments to acquire the phase plot and the zeta potential distribution. Synthesis: Synthesis of Char. In a typical procedure, a mixture of green head of eggplant (10 g), H2SO4 (0.5 ml) and distilled water (100 ml) was prepared in an autoclave and


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then heated at 200 °C for 15 h. After cooling to the room temperature, the black solid was collected by filtration, washed with distilled water and dried at 100 °C for 24 h. According to CHN analysis, the percent of N, C and H was calculated to be 1.62, 45.76 and 5.22%. Noteworthy, the study of the effect of the used amount of green head of eggplant on the content of the Char of the final Hal-Char nanocomposite showed that there was not a linear relationship between the amount of the used green head of eggplant and the Char content on the final nanocomposite. More precisely, TG analysis of various Hal-Char samples prepared by use of different ratio of Hal: head of eggplant (use of Hal: Head of eggplant ratio of 1:2, 1:4 and 1: 8) revealed (Figure S1) that increasing the head of eggplant from 2 to 4 g did not increase the Char content on the resulting Hal-Char nanocomposite, while further increase to 8 g led to the increase of the Char content from 20 wt. % to 26 wt.% on the Hal-Char nanocomposite. As the catalyst prepared by use of 1:4 ratio of Hal: head of eggplant showed the best catalytic results, this ratio was selected as the optimum amount for the preparation of all the control samples.


10

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Synthesis of Hal-Char composite. Inspired by the previous reports,2, 30-31 Hal-Char nano composite was prepared. Typically, a mixture of Hal (1 g), green head of eggplant (4 g), H2SO4 (0.5 ml) in distilled water (100 ml) was prepared in an autoclave. The resulting mixture was then heated to 200 °C for 15 h. Upon completion of the reaction, the resulting dark grey precipitate was separated, washed with distilled water and dried at 100 C. Noteworthy, the control Hal-Char samples with different Char contents were synthesized similarly, except 2 and 8 g green head of eggplant were applied per 1 g Hal. Synthesis of Hal-Char-IL. Initially, the IL was prepared according to the previously reported procedure with slight modification.1 Briefly, N‐methylimidazole (1 mol) was mixed with (3‐chloropropyl)trimethoxysilane (1 mol) in solvent-free condition at 95 °C overnight. Upon completion of the reaction, the mixture was cooled, washed with diethyl ether and dried under vacuum at 40 °C. Subsequently, IL-modified HalChar was prepared as follows: 1.5 g of Hal-Char was added to a solution of IL (1.5 g) and triethylamine (5 ml) in a mixture of water and ethanol as a solvent (50 ml,


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1:1). Then, the resulting mixture was refluxed at 95 °C for 48 h. At the end of the reaction, the as-prepared Hal-Char-IL was washed with distilled water and ethanol several times and dried at 100 C. Synthesis of Hal-Char-SO3H. The modification of Hal-Char by sulfuric acid was performed according to the previous reports with slight modification.32 Typically, 1 g of Hal-Char was added to 10 mL H2SO4 (98%) and mixed at 100 C for 15 h. After that, the acid-treated composite was collected, washed with deionized water several times and dried in oven at 100 C for 12 h. Synthesis of Hal-Char-CD. Typically, 20 ml of NaOH (10 M) was added into a solution of 1.5 g of Hal-Char in ethanol: water (50 ml, 1:1). Subsequently, 1.5 g of tosylated -CD (-CD -OTs) was added to the previous mixture. The latter was then refluxed at 90 °C overnight. Finally, the modified Hal-Char was collected by filtration and dried at 100 °C for 24 h. Synthesis of Pd@Hal-Char. Immobilization of palladium nanoparticles was achieved through wet impregnation procedure. Briefly, 1.2 g of Hal-Char was


12

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added to 30 mL toluene and sonicated for 30 min to furnish a well-dispersed HalChar suspension. Then, a solution of 0.09 mmol of Pd(OAc)2 in 12 mL toluene was added dropwise to the Hal-Char suspension. The mixture was then stirred at room temperature for 2 h. To reduce palladium salt and furnish Pd(0) nanoparticle, a solution of NaBH4 in H2O (10 mL, 0.2 N) was added into the suspension in a dropwise manner and the resulting mixture was stirred for 2 h. Finally, the precipitate was filtered, washed three times with MeOH and toluene and dried in oven at 100 °C for 12 h. The general synthetic procedure of the catalyst is shown in Figure 1. Synthesis of Pd@Hal, Pd@Char, Pd@Hal-Char-IL, Pd@Hal-Char-CD, Pd@HalChar-SO3H, Pd-CD@Hal-Char. The procedure for the immobilization of Pd nanoparticles was similar to the method used for the synthesis of Pd@Hal-Char, except different supports were used instead of Hal-Char.

Noteworthy, for the

synthesis of Pd-CD@Hal-Char, -CD was used as a capping agent in the course of Pd immobilization. Pd@CD was also achieved by reduction of Pd salt in the


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presence of -CD in the aqueous media. In Figure 1, the schematic procedures for the synthesis of the catalysts are depicted.

Figure 1. The schematic procedures for the synthesis of the catalysts.

General procedure for the hydrogenation of nitro compounds. A typical procedure used for the hydrogenation of nitro compounds was as follows: nitro compound (1 mmol), the catalyst (0.03 mol %), the catalytic amount of β-CD (0.004 mmol) and deionized water (2 mL) were placed into a reactor at room temperature. In the course of hydrogenation reaction, hydrogen (1 bar) was purged into the flask and


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the mixture was stirred vigorously (800 rpm). Upon completion of the reaction (monitored by TLC), the catalyst was filtered, washed with H2O and dried at 100 °C overnight for reusing in the consecutive reaction runs. On the other hand, the product was isolated by evaporation of the solvent and identified by comparing its melting/boiling point and FTIR spectrum with that of the authentic sample. RESULTS AND DISCUSSION

The FTIR spectra of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-Char-SO3H are depicted in Figure 2. The FT-IR spectrum of Char showed five main absorption bands at around 3426 cm-1 (-OH), 2923 cm-1 (-CH), 1696 cm-1 (-C=O), 1583 cm-1 (-C=C) and 1284 cm-1 (-C-O). The observation of these bands can confirm that Char contains various functional groups.

Hal-Char spectrum exhibited the characteristic bands of Char. Moreover, an additional bands can be observed at 1081 cm-1 that can be assigned to the Si–O stretching and confirming incorporation of Hal in the structure of the hybrid system. Comparing the FTIR spectrum of the catalyst with that of pristine Hal (to avoid complexity, the FTIR spectrum


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of Hal is presented in SI, Figure S2) showed that some of the characteristic bands of pristine Hal were missing in the FTIR spectrum of Hal-Char. This observation that is in good agreement with other reports 33 can be assigned to the acid treatment in the course of formation of Hal-Char, i.e. use of sulfuric acid. In more detail, in the FTIR spectrum of Hal-Char the bands at 3691 cm-1 and 3624 cm-1 that are representative of inner hydroxyl stretching vibrations of Hal aluminol groups are less pronounced, implying the removal of alumina layer of Hal. Moreover, the characteristic band of Si-O-Al perpendicular stretching (754 cm-1) is disappeared.

The FTIR spectrum of Hal-Char-IL showed the characteristic bands of Hal-Char composite. Furthermore, the characteristic bands of IL, i.e. the bands at 1631 cm-1 (-C=N) and 1576 cm-1 (-C-N) overlapped with the bands of Hal. As depicted, the FTIR spectrum of Hal-Char-SO3H is distinguished from the other samples. In this FTIR spectrum, the bands at 3691 cm-1 and 3624 cm-1 are completely disappeared and a broad band at that region can be observed, indicating the effecting Hal etching and the formation of amorphous spheroidal nanoparticles of SiO2.34 another proof for more effective Hal


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etching is increase of the intensity of the band at1081 cm-1 that is indicative of silica nanoparticles 33. Moreover, the band at 1345 cm-1 can be attributed to -S=O functionality. The FTIR spectrum of Hal-Char-CD exhibited the characteristic bands of Hal-Char composite. The characteristic bands of -CD (-OH, and –CH2) were overlapped with those of Hal- Char. Notably, FTIR spectra cannot solely confirm the structures of the catalysts. Hence, more characterizations were performed to verify the formation and structures of the catalysts.

Figure 2. FTIR spectra of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-CharSO3H.


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Next, the textural properties of the as-synthesized supports and the effect of surface treatment of Char were determined from nitrogen physisorption analysis. The BET specific surface areas of all samples were obtained and compared in Table 1. As tabulated, the specific surface area of Hal-Char composite was 20 m2.g-1 (Table 1, entry 3), that is lower than bare Hal (51 m2.g-1) and almost similar to that of Char (22 m2.g-1). This observation can imply that Char could cover the Hal surface. Moreover, it can be seen that incorporation of IL and -CD could decrease this value to 11 m2.g-1and 19 m2.g-1 respectively, indicating that the surface of the composite was covered with these functionalities. In the case of Hal-Char-SO3H, it can be seen that the acid treatment led to the increase of the specific surface area to 36 m2.g-1. This observation is in good accordance with the literature, in which it is declared that the acid treatment of Hal could result in the increase of the specific surface area.34 Noteworthy, nitrogen adsorption– desorption isotherm of Hal-Char (Figure S3) showed the type II isotherms with H3 hysteresis loops.35


18

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Table 1. Textural properties of the prepared materials Entry

Catalyst Support

SBET (m2.g-1) a

1

Hal

51

2

Char

22

3

Hal-Char

20

4

Hal-Char-IL

11

5

Hal-Char-SO3H

36

6

Hal-Char-CD

19

a

SBET (BET surface areas)

To further characterize the supports and investigate the mass loading of Char, ionic liquid and cyclodextrin, thermogravimetric analysis of the supports was performed. As illustrated in Figure 3, the thermogram of Char showed two weight loss stages, the first one below 200 °C is related to the loss of water and the next one at 580 °C can be assigned to the degradation of Char. The thermogram of Hal-Char exhibited three main weight losses, at about 150, 500 and 580 °C. The first weight loss stage is due to the loss of water in the structure of Hal and Char, the other weight losses are due to the


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dehydroxylation of the Hal matrix and decomposition of Char respectively.36 To measure the content of char, the thermogram of Hal-Char was compared with that of the pristine Hal (to avoid complexity, the thermogram of Hal is presented in Figure S4). It was found that the content of Char was about 20 w/w%. Considering the thermogram of Hal-CharIL, four weight loss stages over the range of 50-800 °C can be seen. The first stage at about 130 °C is due to the loss of water, the second one at about 300 °C is due to the IL decomposition37 and two others can be assigned to the decomposition of Char and dehydroxylation of the Hal. Comparing this thermogram with that of Hal-Char, the content of IL was estimated to be about 7 wt. %. Considering the thermogram of Hal-Char-CD and comparing it with that of Hal-Char, it can be found out that the content of CD on the Hal-Char-CD was very low (about 2 wt.%). In the case of Hal-Char-SO3H, the thermogram is different from other samples. Indeed, the first weight loss stage is occurred at 150 °C that is due to the loss of water and the two other ones at 330 and 550 °C are due to the degradation of organic moieties and Char. As discussed above, this observation can be attributed to the dissolution of inner surface of Hal and the formation of amorphous spheroidal nanoparticles of SiO2.34


20

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Figure 3. The TG analyses of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-CharSO3H.

The prepared catalyst supports were also characterized via XRD analysis (Figure 4). The XRD pattern of Char exhibited a sharp band (002) at 25° and a broad band (ɣ) around 15°. According to the literature,38 the (002) band indicates the graphite-like structures (crystalline carbon), while the other band is due to the saturated structures such as aliphatic side chains, attached to the edge of the crystalline carbon.38 The XRD pattern of Hal-Char exhibited the characteristic bands of Char and the bands at 2θ = 16.4°, 17.9°, 25.5°, 26.6°, 29.8°, 39.3°, 52.2° and 61.6°. The comparison of the XRD pattern of HalChar with that of pristine Hal (to avoid the complexity, the XRD pattern of pristine Hal is


21


depicted in Figure S5

35, 39),

Page 22 of 54

it can be seen that two XRD patterns are distinguishable.

More precisely, the intensities of the two first characteristic bands of Hal was decreased. According to the literature

33,

this observation can be due to the partial loss of the tube

wall crystallinity. The comparison of the XRD patterns of the Hal-Char-IL, Hal-Char-CD with that of Hal-Char showed that the incorporation of ionic liquid and -CD did not induce significant change. More precisely, neither significant shift in the bands nor change of the interlayer distance was observed in the XRD patterns of these two samples compared to that of Hal-Char. Contrary to the mentioned samples, the XRD pattern of Hal-Char-SO3H indicated that the acid modification of Hal resulted in a significant change in the structure of the catalyst and the intensities of the characteristic bands of Hal reduced dramatically. According to the literature,34 long term treatment of Hal with concentrated acid can lead to delamination in an acidic environment and formation of amorphous silica.


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Figure 4. XRD patterns of Char, Hal-Char, Hal-Char-IL, Hal-Char-CD and Hal-CharSO3H.

The XRD analysis was also exploited for the characterization of Pd@Hal-Char catalyst (Figure 5). As shown, the XRD pattern of the catalyst is similar to that that Hal-Char (to provide better comparison, the XRD pattern of Hal-Char is merged in Figure 5) and the characteristic bands of Pd nanoparticles are not observable. According to previous reports, this issue can be assigned to the low loading of Pd, as was also confirmed by ICP analysis, and its high dispersion, as detected in TEM images.40


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Figure 5. XRD patterns of Hal-Char and Pd@Hal-Char.

The nitrogen adsorption–desorption isotherm of the Pd@Hal-Char catalyst was also recorded (Figure 6) and compared with that of Hal-Char catalyst (Figure S3). The comparison showed that both samples exhibited the same type of isotherm (type II isotherms with H3 hysteresis loops). Comparison of the specific surface area of two samples revealed that upon immobilization of Pd nanoparticles, the specific surface area reduced to 15 m2.g-1, confirming that Pd nanoparticles are located on the surface of the support.


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Figure 6. Nitrogen adsorption–desorption isotherm of the Pd@Hal-Char catalyst.

To further characterize Pd@Hal-Char, it was subjected to the FTIR and TG analyses (Figures 7 and 8). As shown in Figure 7, the FTIR spectrum of the catalyst is similar to that of Hal-Char.


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Figure 7. FTIR spectrum of Pd@Hal-Char.

Similar to the thermogram of Hal-Char, Pd@Hal-Char thermogram (Figure 8) showed three weight loss steps, (150, 500 and 580 °C) related to the weight loss of water, dehydroxylation of the Hal matrix and decomposition of Char respectively.


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Figure 8. Thermogram of Pd@Hal-Char.

To study the dispersion, the morphology and the average size of Pd nanoparticles in the catalysts, the TEM analysis of these catalysts has been performed at low and high magnification (Figure 9 and Figure S6). To elucidate how use of Hal, Char and the modified Hal-Char supports affect the distribution and average Pd particle size, the TEM images of Pd@Hal-Char and all of the control catalysts were recorded (Figures 9 and Figure S6) and compared with that of the catalyst. According to the previous reports

33,

H2SO4 could dissolve alumina sheets in the structure of Hal. This phenomenon, i.e. Hal etching, that occurs through diffusion of hydrogen ions within Hal pores and following


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interactions with alumina, is dependent on the reaction time, temperature and H2SO4 concentration. Hence, it was expected that the morphologies of the catalysts were distinguished from pristine Hal and on the other hand, Pd@Hal-Char-SO3H that was prepared through harsher acidic condition differed from other catalysts. As expected, in the TEM images of Pd@Hal-Char, Pd-CD@Hal-Char, Pd@Hal-Char-CD, Pd@Hal-CharIL, that were treated with more diluted acidic solution for shorter reaction time, the rodlike structure of Hal was observable. However, small SiO2 nanoparticles, derived from dealuminated Hal can also be detected. In the case of Pd@Hal-Char-SO3H that was treated with sulfuric acid twice, first in the step of Hal-Char synthesis and secondly in the course of introduction of –SO3H functionalities, aggregates of SiO2 nanoparticles can be detected, indicating more etching of Hal. In other word, due to the effective etching, Hal tubular morphology changed to porous nonorods. Notably, alumina etching can result in the enlargement of lumen diameter, especially in the case of Pd@Hal-Char-SO3H.

The TEM images of the catalyst also showed that Hal is covered with Char. Moreover, the small dark spots on the Hal-Char are representative of Pd nanoparticles. As can be


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seen, the Pd nanoparticles are well-dispersed on the surface of Hal-Char and no aggregation of Pd nanoparticles was detected, implying the efficiency of Hal-Char as a support for the immobilization of Pd nanoparticles. Crystalline domains are shown on the TEM images of Pd@Hal-Char at magnification of 390K with the presence of distinct lattice planes, which can be further analyzed by using reduced Fast Fourier Transform (FFT)derived diffraction patterns (see Supporting Information, Figure S7). The FFT-derived diffraction pattern indicates that the diffraction spot could be identified as the Pd nanocrystals with a face-centered-cubic (fcc) structure, based on the reflection from the (111) planes having a typical d-spacing of 0.23 nm.


29


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a


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31


20

f

Ø m= 4.5  1.1 nm

15

%

10 5

1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5

0 20 nm

Diam eter (nm)

25

g

Ø m= 3.9  1.0 nm

20 15 %

10 5 0 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 54

20 nm

Diam eter (nm)

b

Figure 9. a) TEM images at a magnification of 150K of Pd@Hal-Char and b) The TEM images and the corresponding size distributions of (a) Pd@Hal-Char ; (b) Pd@Char ; (c) Pd@Hal ; (d) Pd-CD@Hal-Char ; (e) Pd@Hal-Char-CD ; (f) Pd@Hal-Char-IL and (g) Pd@Hal-Char-SO3H catalysts.

Next, the activity of the Pd@Hal-Char catalyst in the hydrogenation of nitrobenzene and 1-nitronaphthalene as two model substrates in the presence of H2 gas was investigated (Scheme 1). Initially, the model reactions were carried out under mild and


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environmentally-benign condition, i.e. in the presence of 0.03 mol %Pd@Hal-Char at 50 °C in water as solvent and under 1 bar of hydrogen.

NO2

NH2 Pd@Hal-Char CD NO2

H2

NH2

Scheme 1. Hydrogenation of nitroarene and 1-nitronaphthalene. Under these conditions, aniline and naphthylamine with moderate yields were furnished respectively after 1 h. To increase the yields of the reactions and decrease the reaction time, the reaction variables, i.e. the solvent, the catalyst amount and the reaction temperature were optimized. It was indicated that at lower temperature (room temperature), the reaction could proceed to furnish the desired products with comparative yields. Hence, 25 °C was selected as the reaction temperature. Moreover, it was found that the reaction could be catalysed in the presence of lower amount of the catalyst, 0.03 mol %Pd@Hal-Char, and further increase of the catalyst content did not improve the yield of the reaction. Furthermore, water was found as the chosen solvent. Under the optimum


33


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reaction conditions, the model reactions led to the formation of aniline and naphthylamine in 75 % and 65% yields respectively after 1 h. To further increase the yields of the products, β-CD (0.004 mmol) was utilized as phase transfer agent along with the catalyst. Interestingly, it was found that the use of the catalytic amount of CD increased the reaction yields significantly.

Table 2. The catalytic activity, the average Pd particle size and the loading of the catalytic samples.a

Aniline Entry

Catalyst

Yield (%)b

Naphthylamine Yield (%)b

Pd loading (mmol.g-1)

Pd NPs size (nm)

TOF for Aniline (h-1)

TOF for Naphthylamine (h-1)


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1


Pd@Hal

30

10

0.027

4.8±1. 1297

307

2810

1844

1307

693

1951

1156

3 2

Pd@Hal + -CDc

65

60

0.027

4.8±1. 3

3

Pd@Char

40

30

0.026

3.6±1. 1

4

Pd@Char + -CD

75

50

0.026

3.6±1. 1

5

Pd@Hal-Char

75

65

0.028

3.7±1

2435

1490

6

Pd@Hal-Char + -CD

100

95

0.028

3.7±1

3226

2179

7

Pd@Hal-Char-ILd

60

30

0.041

4.5±1. 1612

570

2406

834

1 8

Pd@Hal-Char-IL + -CD

90

45

0.041

4.5±1. 1

9

Pd@ Hal-Char-SO3He

45

50

0.028

3.9±1

1524

1204

10

Pd@ Hal-Char-SO3H + -CD

55

55

0.028

3.9±1

1863

1324

11

Pd@ Hal-Char-CD

60

50

0.025

4.9±1. 2662

1696

7


35


12

Pd-CD@ Hal-Char

50

40

0.032

Page 36 of 54

3.8±1

1451

825

Reaction condition: Substrate (1 mmol), catalyst (0.03 mol%), -CD (0.004 mmol), H2O (2 mL), H2 (1 bar) at 25 C in 1 h. a

b Isolated c

yields

Use of β-CD in its free form

d Trimethoxysilyl e

propyl imidazolium ionic liquid (IL) grafted on Hal-Char Functionalized Hal-Char using H2SO4

Armed with the optimum reaction conditions, contribution of Hal, Char and -CD to the catalysis was scrutinized. First, to investigate whether combination of Hal and Char could affect the catalytic activity, two control catalysts, Pd@Hal, Pd@Char, were synthesized and their catalytic activities were compared with that of the catalyst (Table 2, entries 1 and 3). The precise comparison of the catalytic activity of the catalyst with that of Pd@Hal and Pd@Char showed that the catalytic activity decreased in the order of Pd@Hal-Char > Pd@Char > Pd@Hal, confirming that the use of Hal-Char nanocomposite as a catalyst support was more efficient than the use of each component individually. To elucidate the origin of this observation, the Pd loading (obtained from ICP) and the average Pd particle size (obtained from TEM) for these three samples were compared (Table 2, entries 1, 3 and 5). As tabulated, in all three samples the loading of Pd is almost similar. More


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precisely, the Pd loading in the catalyst is slightly higher than those two other samples, while the Pd loading in Pd@Char was the lowest. Considering the average Pd particle size and the TEM images, it can be seen that in the case of Pd@Hal, the average Pd particle size was the largest (4.8 ± 1.3 nm) and the Pd nanoparticles were not welldispersed on the support, while in Pd@Char and Pd@Hal-Char, the Pd nanoparticles were highly dispersed on the support and the average Pd particle sizes were almost similar and smaller than that of Pd@Hal. Considering the difference of the catalytic activities of Pd@Hal and Pd@Char that possess almost similar Pd loading but different Pd average particle sizes, it can be concluded that the Pd average size can effectively affect the catalytic activity. Moreover, it can be found that Char as a support can lead to the effective dispersion of the Pd nanoparticles compared to Hal. Comparing the catalytic activity of the catalyst and Pd@Char that has almost similar Pd loading and average Pd particle sizes, it can be postulated that the incorporation of Hal can improve the catalytic activity probably through the synergistic effect with Char. In order to better understand these different catalytic results, the zeta potential measurements of these catalysts dispersed into aqueous media have been carried out. Indeed, zeta potential


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measurements are used to characterize the stability of colloidal suspensions into solvents. Then, it should be noticed that in our case, there is a correlation between the zeta potential value of the catalyst into water and the catalytic activity which could be related to a better dispersion. These measurements are summarized in Table S1 and Figure S8. The zeta potential value of the pristine Hal is about -32.9 mV. This value is commonly reported in the literature.41 The zeta potential values of Pd@Hal-Char, Pd@Char and Pd@Hal are -53.0 mV, -50.1 mV and -40.3 mV respectively, indicating that the colloidal dispersion stability is decreasing in the order Pd@Hal-Char > Pd@Char > Pd@Hal. Consequently, there is a good correlation between the colloidal suspension stability and the catalytic activity. Indeed, the better the colloidal dispersion stability, the lower the aggregation of the particles and consequently the better interactions between the substrate and the catalytic species would be. Next, to further confirm the role of -CD in the catalysis, the catalytic activities of the two aforementioned control catalysts Pd@Hal and Pd@Char in the presence and absence of -CD were examined. It was shown that (Table 2, entries 2 and 4) in all cases, the use of -CD as a co-catalyst could significantly increase the yields of the reactions. Notably, the amount of -CD is an important


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parameter and the use of lower quantity of -CD led to lower yields of the desired products. The contribution of -CD to the catalysis can be attributed to its capability of formation of inclusion complex with the hydrophobic substrate, nitroarene and 1-nitronaphthalene, and transfering them into the aqueous media.18 To elucidate whether the incorporation of -CD in the scaffold of the catalyst could affect the catalytic activity as effectively as -CD in its free form, a control catalyst, Pd@HalChar-CD, was synthesized and used as a catalyst for promoting the model reactions. Interestingly, it was found that (Table 2, entry 11) the incorporation of -CD in the structure of the catalyst was not as effective as the use of -CD in its free form and the use of CD as a co-catalyst led to a catalytic system with superior catalytic activity. Comparing the Pd loading and the average Pd particle size of the catalyst with those of Pd@HalChar-CD, it can be found that the Pd loading in the catalyst was slightly higher than that of Pd@Hal-Char-CD, while, the average Pd particle size in the catalyst was smaller than that of Pd@Hal-Char-CD (4.9±1.7 nm). This observation further confirmed the role of the Pd nanoparticle size on the catalytic activity. On the other hand, the lower catalytic activity of Pd@Hal-Char-CD can be justified by considering the amount of -CD in the two


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reaction mixtures. As discussed in the characterization section, the amount of -CD in Pd@Hal-Char-CD sample is only 2 wt.%. Hence, the total content of -CD in the reaction mixture would be much lower than that of Pd@Hal-Char, in which 0.004 mmol β-CD was used as co-catalyst. This issue confirms the importance of the role of -CD as phase transfer agent. Even if the zeta potential value of Pd@Hal-Char-CD is higher than Pd@Hal-Char catalyst, it should be noticed that, considering the zeta potential distribution of Pd@Hal-Char-CD (Figure S8), there is an important modification of the total counts and a broadening of the zeta potential distribution shape during the triplicate analysis. Consequently, the exploitation of zeta potential measurement in the case of Pd@HalChar-CD is not so clear. It is postulated that in the Pd@Hal-Char-CD, low content of -CD in the catalyst structure prevented it from acting properly as a phase transfer agent. To validate this assumption, a control reaction in the presence of Pd@Hal-Char-CD and -CD was carried out. It was found that the addition of -CD could significantly improve the yields of the desired products.


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To investigate whether the use of -CD as a capping agent in the course of Pd immobilization could improve the catalytic activity, another control catalyst, Pd-CD@HalChar, was prepared and its catalytic activity was compared with that of Pd@Hal-Char. Lower catalytic activity of this sample (Table 2, entry 12) compared to that of the catalyst was observed. This difference is in correlation with zeta potential measurement and also colloidal suspension stability in water. Indeed, the zeta potential value of Pd-CD@HalChar is about -32.2mV and also lower than Pd@Hal-Char zeta potential value. Moreover, the comparison of the structural features of Pd-CD@Hal-Char with that of the catalyst implied that the Pd loading in the former is higher than the catalyst, while Pd nanoparticles in both samples were well-dispersed and the average Pd particle sizes in both samples were almost similar. Considering these results, it was expected that Pd-CD@Hal-Char exhibited superior catalytic activity compared to Pd@Hal-Char. However, converse result was obtained and the catalytic activity of Pd-CD@Hal-Char was much lower than that of the catalyst. Similar to Pd@Hal-Char-CD, the lower catalytic activity of Pd-CD@Hal-Char can be attributed to the low amount of -CD in the sample that did not allow the effective phase transfer of the hydrophobic substrates. It is worth mentioning that the use of -CD


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in its free form along with Pd-CD@Hal-Char could remarkably increase the yield of the reaction, implying the crucial role of phase transfer of substrates during the catalytic process. Noteworthy, the investigation of the catalytic activity of Pd@CD as a homogeneous catalyst confirmed its inferior catalytic activity compared to that of Pd@HalChar (45 and 40% aniline and naphthylamine were achieved respectively). The effect of the surface modification of Char on the catalytic activity was also investigated. To this purpose, two control catalysts, Pd@Hal-Char-IL and Pd@Hal-CharSO3H were prepared and their catalytic activities for promoting the model reactions in the presence and absence of -CD were compared with that of the catalyst. It was proved that (Table 2, entries 7-10) in these two cases, the use of -CD is essential for achieving higher yields. Moreover, it was found that the surface modification of Char with IL or – SO3H functionalities had detrimental effect on the catalytic activity. Moreover, the zeta potential values of Pd@H-C-IL and Pd@H-C-SO3H were calculated to be -23.4 mV and -39.1 mv respectively, that were lower than that of Pd@Hal-Char. Considering these results, the lower catalytic activities of Pd@H-C-IL and Pd@H-C-SO3H can be attributed to their lower dispersion into the aqueous phase. The comparison of the Pd loading in


42

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Pd@Hal-Char-IL with that of the catalyst proved the higher Pd content in Pd@Hal-CharIL. This observation can be attributed to the role of IL in anchoring the Pd nanoparticles through providing electrostatic interactions.42 Although Pd nanoparticles are the main catalytic active sites in the hydrogenation reaction, it can be seen that this sample with higher Pd content exhibited lower catalytic activity than the catalyst that has lower Pd loading. This observation can be justified by considering the larger average Pd particle size in Pd@Hal-Char-IL (4.5± 1.1 nm) compared to that of the catalyst. In the case of Pd@Hal-Char-SO3H, the Pd loading and the average Pd particle size were almost similar to those of the catalyst, however, its catalytic activity was lower than that of the catalyst. This observation can be assigned to the dissolution of Hal in the course of acid treatment. This result can further confirm the role of Hal in the catalysis and the synergism between Hal and Char. It was found that the Pd@Hal-Char catalyst could efficiently promote the hydrogenation of nitrobenzene and 1-nitronaphthalene that is a sterically demanding substrate. To further investigate the generality of this protocol and study whether the catalyst was selective toward the nitro functional group and could selectively hydrogenate –NO2 over


43


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–C=O, the hydrogenation of 4-nitroacetophenone in the presence of Pd@Hal-Char and -CD was also carried out. It was demonstrated that under the reaction condition, the corresponding product could be achieved in quantitative conversion and yield (100%) and no alcohol was detected as by-product, confirming the selectivity of the catalyst.

One of the most important advantage of heterogeneous catalysts is their capability of recovery and recycling. In this context, the stability of Pd@Hal-Char upon recycling was examined. To this purpose, upon completion of the reaction, Pd@Hal-Char was filtered off, washed with water for several times and then dried at 100 °C and subjected to the next reaction run. The yield of the desired product after each recycling was obtained and compared with that of fresh Pd@Hal-Char (Figure 10). As illustrated in Figure 10, Pd@Hal-Char exhibited high recyclability up to 7 runs without significant loss of its catalytic activity. As shown in Figure 11, the FTIR spectrum of the recycled catalyst after seven runs showed the characteristic bands of the fresh Pd@Hal-Char. The similarity of the FTIR spectra of the fresh and recycled catalysts indicated the structural stability of Pd@Hal-Char upon recycling. Moreover, the FTIR spectrum of the recycled catalyst did


44

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not show additional bands, implying that the active surface of the catalyst was not covered with organic substrates. All of these characterizations clearly show the robustness of Pd@Hal-Char catalyst during the catalytic process.

Figure 10. Recyclability of Pd@Hal-Char for the hydrogenation reaction of nitrobenzene under optimum reaction conditions.


45


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Figure 11. FTIR spectra of the recycled Pd@Hal-Char catalyst, Blue: fresh and Red: recycled catalyst after seven recycling.

CONCLUSION

In summary, Hal-Char nanocomposite was prepared through easy hydrothermal procedure using green head of eggplant as a Char source. The nanocomposite was then successfully applied for the immobilization of Pd nanoparticle and development of Pd@Hal-Char catalyst that along with -CD co-catalyst exhibited high catalytic activity and recyclability for the hydrogenation of nitroarenes under


46

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mild and eco-friendly conditions. In the next part of this research, a precise study has been carried out to elucidate the contribution of Hal and Char to the catalysis. Moreover, the effect of Char surface modification with acid treatment and IL incorporation on the catalytic activity and the role of -CD in the catalysis and the effect of using it in its free form and incorporated in the backbone of the catalyst on the catalytic performance were studied. To this purpose, several control catalysts, including Pd@Hal, Pd@Char, Pd@Hal-Char-CD, Pd-CD@Hal-Char, Pd@CD, Pd@Hal-Char-IL, Pd@Hal-Char-SO3H were prepared and their catalytic activities were compared with that of the catalyst. Moreover, the catalytic activities of the catalysts in the absence and presence of -CD were investigated. It was confirmed that the use of -CD as phase transfer agent along with the catalyst is imperative to achieve high catalytic activity. Furthermore, comparison of the Pd particle average size and the loading of all the samples demonstrated that the content of -CD in the reaction mixture played an important role in the catalysis and in the Pd@Hal-Char-CD and Pd-CD@Hal-Char samples that -CD content was not adequate to allow effective formation of inclusion complex with hydrophobic substrates, low catalytic activities


47


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were observed. Moreover, the catalytic activities of Pd@Hal and Pd@Char were inferior to Pd@Hal-Char, indicating the synergism between Hal and Char. Regarding the surface modification of Char, it was found that incorporation of IL led to higher Pd loading but larger average particle size and consequently lower catalytic activity. In the case of acid treatment, the analyses confirmed that Hal could be structurally altered under acidic conditions. This issue had a detrimental effect on the catalytic activity.

AUTHOR INFORMATION

Corresponding Author *E-mail : [email protected], [email protected]

ORCID Samahe Sadjadi : 0000-0002-6884-4328 Bastien Léger : 0000-0003-2411-1162 Eric Monflier : 0000-0001-5865-0979 Majid M. Heravi : 0000-0003-2978-1157

ACKNOWLEDGMENTS


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The TEM in Lille (France) are supported by the Conseil Régional du Nord-Pas de Calais and the European Regional Development Fund (ERDF). The authors are grateful to Dr. Ahmed Addad (UMET, University of Lille) for technical assistance in TEM measurements. This work has been supported by the Center for International Scientific Studies & Collaborations (CISSC) and French Embassy in Iran and Hubert Curien French-Iranian partnership “PHC GUNDISHAPUR 2018” n° 40870ZG. MMH and Akbari are also thankful to Iran National Science Foundation for the Individual given grant, No. 96010807. S. Sadjadi and Heravi appreciate Iran National Science Foundation for the Individual given grant, No. 97009384.

ABBREVIATIONS Halloysite : Hal ; Hydrochar : Char ; Halloysite-hydrochar : Hal-Char ; -cyclodextrin : CD ; Ionic liquid : IL

SUPPORTING INFORMATION SUMMARY


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The supporting information provides details on thermogravimetric analyses of pristine Hal and Hal-Char materials with different char contents, FTIR spectrum of pristine Hal, Nitrogen adsorption–desorption isotherm of the Hal-Char, XRD pattern of pristine Hal, TEM images of the samples at low magnification, TEM image at high magnification and reduced FFT-derived diffraction pattern, zeta potential values and zeta potential distribution of pristine Hal and the different catalysts.

AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

REFERENCES

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“For Table of Contents Only” SYNOPSIS Pd immobilized on halloysite-hydrochar nanocomposite has been prepared and applied for hydrogenation of nitroarenes in the presence of CD.


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Eggplant-Derived Biochar-Halloysite ... - ACS Publications

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